This invention pertains to a drop-on-demand ink jet printing method, more particularly to a method of printing wherein a purge image is logically combined with a selected image to insure a desired amount of drop firing from every jet of an ink jet printhead for every page printed. The inventive method avoids image defects that could otherwise occur as a result of faulty drop firing from infrequently used nozzles.
Drop-on-demand ink jet printing is a non-impact printing process in which droplets of ink are deposited on print media, such as paper, to form the desired image. The droplets are ejected when needed (demanded) from a printhead in response to electrical signals generated by a microprocessor and are directed to specific locations (pixel positions) on the print media. The printhead and print media are moved relative to each other while drops are ejected so that all pixel positions are traversed along scanlines in the direction of movement. In some ink jet printers a page wide array (PWA) printhead, as wide as the entire image area to be printed and having a sufficient plurality of jets to deposit drops on every image scanline perpendicular to the direction of relative motion, is employed. In other ink jet printers a narrow printhead is scanned in a main scan direction and the medium is advanced in a perpendicular sub-scan direction in increments of image scanlines.
Drop ejection performance suffers as the time interval between drop ejections increases. Because drops are ejected only when “demanded” or needed to form the selected image, there are varying amounts of time between jet firings for each of the jets or nozzles in the printhead based on the density of required drops along the scanline addressed by each jet. Some jets may traverse image areas having all “white” space and so be required to print no drops for one or more full pages of images. Other jets may print almost continuously because they align with lines in the image that run the length of the image area. If a jet remains idle, it has a tendency to become plugged or clogged as a result of ink vehicle evaporation and crusting of the ink or dye precipitation out of the ink in or around the jet, which can result in the formation of a viscous plug in the jet orifice. If a jet has plugged, ink droplets ejected through the jet orifice will be misdirected, which will adversely affect print quality.
Substantial variations in drop volume and ejection velocity, i.e., more than 20% from nominal values, usually results in noticeable image defects in the form of image noise and line raggedness. More severe variations may cause stuttering ejection, non-ejection and misdirection of drops to the point that visible light and dark streaks are formed in the image.
The inventive methods disclosed herein counteract degradation in performance caused by ink evaporation from infrequently used nozzles.
It has long been known and practiced in drop-on demand ink jet printing systems to cause the ejection of non-printing drops in order to restore jet performance. This procedure, commonly termed purging, requires the nozzle to spit on a regular basis into a waste container (spittoon) to expel ink in the nozzle region that has been evaporatively degraded, refilling the jet with fresh ink having the design intention properties, especially ink viscosity and surface tension, which are needed for nominal drop ejection performance.
Most ink jet printing systems in use today are configured with relatively narrow printheads that are repeatedly scanned over the print medium by a carriage mechanism. One or more stationary locations, spittoons, outside of the print medium area are provided to receive non-print drops during purging. Jet performance is maintained satisfactorily as long as the time required to traverse the medium and reach a spittoon, Ts, is not so long as to allow an unacceptable amount of ink vehicle evaporation in the nozzle. The time a drop-on-demand ink jet may be held in a waiting state, before firing a print drop having nominal velocity and volume, is commonly referred to as the latency (or decap) time, Tl. Thus, for ink jet printers that rely on purging into a spittoon, the design of system elements preferably are balanced so that Tl>Ts. The relevant system elements include the ink jet printhead drop ejection process, ink flow path, nozzle region geometry, ink formulation, printhead temperature range, carriage motion profile, location of the spittoon, width of the print zone, and environmental factors such as temperature, relative humidity and elevation.
While spittoons have been successful in many ink jet printing systems, spittoon purging cannot effectively be employed wherein the print media is uninterrupted along the direction of relative motion, as occurs when printing on print media webs or product materials with stationary printheads. Even in the case of moving carriage mounted printheads writing across the media in a main scan and reaching a spittoon or cap location to the side, the width of the media may become so large that the spittoon access time, Ts, becomes very large, thereby imposing difficult constraints on ink formulation materials. Very wide carriages, wider than 2 meters, may be used in textile printers and for printers used for large signage applications, leading to much larger spittoon access times than are typical for letter-size media printers. In addition, spittoon purging apparatus must be designed to contain significant volumes of purged ink materials, potentially for the expected life of the machine. Provisions to capture, move and retain purged ink residues frequently result in complex arrangements of multiple porous materials and receptacles. Such purged ink residue handling apparatus are the source of additional reliability problems and present difficulties for the user in self-servicing and refurbishing the printer apparatus. Finally, as ink jet printing has moved to smaller drop volumes for higher image resolution and increased colorant loadings for improved image permanence, the difficulty of achieving large values for ink latency, Tl, have increased, further exacerbating the design difficulties of managing an increase in non-print drop purging requirements during image printing.
Therefore, a method of maintaining a drop-on-demand printhead that does not rely on purging into a spittoon during image printing is necessary for certain applications, such as page wide array (PWA) printing on print media webs, and may be highly advantageous for moving carriage architectures by easing ink formulation restrictions and reducing the size and complexity of purged ink receptacles. To that end, the inventive methods and apparatus disclosed herein achieve printhead maintenance by drop purging directly onto the print medium. That is, in order to assure that all jets are operating within a required latency time, drops may be printed based on printhead maintenance information as well as based on selected image data.
Non-image drop purging onto the print medium has been disclosed in U.S. Pat. No. 5,659,342 issued to Lund, et al., on Aug. 19, 1997, hereinafter denoted as Lund '342. Lund '342 discloses methods whereby all nozzles are purged by firing purging droplets into background portions of a print media page. Lund '342 further discloses randomly distributing purge droplets, spacing purge droplets at least three dot widths away from one another, and using a visible pattern, such as a watermark, logo, pleasing image, or the like. Lund '342, however, does not disclose a method whereby an imperceptible purge image is constructed independently of any user selected image information and in a way to insure that every print image scanline will require at least one printed drop during printing.
U.S. Pat. No. 6,166,828 issued to Yamada, et al., on Dec. 26, 2000, Yamada '828 hereinafter, discloses methods of ink jet printing whereby on-print-media purging is done based on the history of usage of a given jet among the plurality of jets in an ink jet printhead. Previous binary print data for each jet is monitored and multi-level data is added to the user selected multi-level image data prior to binarization for pending printing by the jets of the printhead. Thus, the on-print-media purging method described in Yamada '828 is image data dependent and must be constructed anew for each jet for each user selected image, thereby requiring considerable computational resources within the printer system. Further, since the computation of prior history of usage is practically limited to a small set of alternative results, the method may introduce noticeable structured image defects in the form of spatially repetitive purge drops.
U.S. Pat. No. 6,296,342 issued to M. Oikawa on Oct. 2, 2001, Oikawa '342 hereinafter, discloses apparatus for maintaining an ink jet printhead that jets a colorless processing liquid by ejecting drops to the print medium from any jet that has not been used for a predetermined time period. The disclosed processing liquid is deposited to purposefully mix with and chemically alter colored ink dots that have been jetted by colored ink jets in the printing apparatus. This approach of jetting purge drops of the colorless processing liquid at fixed time intervals, if applied in like manner for colored ink jets, would result in a highly perceptible periodic pattern of purge drops that overlays the user selected image.
U.S. Pat. No. 6,402,292 issued to T. Ninomiya on Jun. 11, 2002, Ninomiya '292 hereinafter, discloses apparatus for maintaining a PWA ink jet printhead that jets a colorless processing liquid used in conjunction with a PWA printhead that jets a colored ink. Ninomiya '292 discloses that a preliminary discharge of purge drops from each jet is needed to assure nozzle cleansing prior to the printing of each image. The colorless processing pre-discharges are done onto the cut sheet print media itself whereas the colored ink pre-discharges are done onto a media transport belt in gap areas between transported cut sheets. While the colorless processing liquid pre-discharges may not be perceptible on the final print, pre-discharges of the colored inks would produce a noticeable ragged line across the lead edge of the cut sheet.
U.S. Pat. No. 6,523,932 issued to E. Johnson on Feb. 25, 2003, Johnson '932 hereinafter, discloses a method of maintaining an ink jet printhead that prints on a continuous web by greatly slowing the web below any print mode speed and jetting purge drops onto the web. This method therefore produces ragged ink lines at waste areas between user selected images on the web and may cause loss of printing throughput as the web is periodically slowed to perform the needed purging.
U.S. Pat. No. 6,896,349 issued to Valero, et al., on May 24, 2005, Valero '349 hereinafter, discloses printing apparatus for maintaining an ink jet printhead by printing purge drops onto a sheet of print media provided specifically for that purpose. The apparatus of Valero '349 prints purge drops onto a cut sheet of media and then diverts that sheet into a holding position from which it can be recycled for a number of purge drop print cycles before it is considered exhausted for this purpose. The purge drops of Valero '349 are not combined with user selected image drops and outputted with the user selected image. The Valero '349 apparatus adds the complexity of an auxiliary media path for the purge receiver sheet and cannot provide purge drops within the timeframe of printing a selected image.
U.S. Pat. No. 7,029,095 describes a preliminary ejection of an ink drop which is less than the normal amount of ink ejected. It is suggested that these low volume ink drops will not be conspicuous on the media. Here the printer controller/computational system must count the time since the last ejecting operation and make a decision whether another ‘preliminary’ ejection of low volume is ejected.
U.S. Patent Application 2006/0214961 describes a preliminary-ejection control method for a plurality of linearly arranged nozzles which periodically ejects ink at a predetermined time during the recording operation of image data, where the ejection of ink is not based on the image data.
U.S. Patent Application 2006/0284922 describes a maintenance method for an array printer. It describes a control unit to control a maintenance operation and the control unit requires accumulating nozzle information presumably in the computer and/or control system for the printer and when the preset reference time is exceeded the spitting operation is performed.
The above noted disclosures of ink jet printhead purging methods and apparatus that print purge drops onto an imaging media are unsatisfactory for reasons of added cost and complexity, generation of noticeable image artifacts, added computational needs, added hardware subsystems, creation of waste, reduction of productivity or inability to purge at time intervals less than a full image print time. Consequently there is a need for ink jet printhead purging apparatus and methods that are responsive to short timeframe purging requirements arising in very high quality ink jet printers using very small drops and short latency inks. Further there is a need for an on-print media purging method and apparatus that can be implemented in a simple fashion and that does not add noticeable image artifacts to user selected images and does not require significant computational capacity.